Conrad Murray, the cardiologist on trial in the death of Michael Jackson, spoke to or received calls from four different girlfriends on the day the pop legend died, and was apparently on the phone with one of them at the moment he discovered Mr. Jackson’s life was in peril.

Sade Anding, a Houston cocktail waitress, testified on Tuesday, the sixth day of Mr. Murray’s involuntary manslaughter trial in Los Angeles, that the doctor called her to chat shortly before noon on June 25, 2009.

The timing of the call is important because Murray had earlier administered an intravenous dose of a powerful sedative to Jackson. Prosecutors say instead of constantly monitoring his patient’s vital signs, Murray assumed Jackson was asleep and used the time to make business and personal calls on his cellphone.

Ms. Anding said she and Murray spoke for about five or six minutes before she realized that the doctor was no longer listening. “I said, ‘Hello. Hello.’ I didn’t hear anything,” she told the jury. She said she pressed her ear tight against the phone.

“I heard mumbling of voices,” she said. “It sounded like the phone was in his pocket or something, and I heard coughing.”

Jackson was later pronounced dead at UCLA Medical Center and an autopsy established the cause of death as a lethal overdose of the anesthetic propofol.

Jackson had been having difficulty sleeping and Murray agreed to treat his insomnia with a combination of sedatives and intravenous doses of propofol.

Medical experts say propofol is not a recognized treatment for insomnia and that the drug should only be administered in a surgical setting like a hospital. They stress that a strong dose could cause the patient to lose the ability to breath on his own. As a result, experts say, propofol should only be administered under constant supervision and with a full array of vital signs monitoring and emergency response equipment.

Defense lawyers admit that Murray was treating Jackson with propofol in the bedroom of his Los Angeles mansion, but they suggest the lethal dose was taken by Jackson himself.

Beyond the issue of timing, testimony from the girlfriends may also be important if the jury takes offense at Murray’s apparent lifestyle. The testimony paints Murray as a middle-aged player with a taste for leggy 20-somethings, who frequented strip clubs and used his association as Michael Jackson’s personal doctor to facilitate his active social life.

Superior Court Judge Michael Pastor has barred prosecutors from introducing information about Murray’s wife and children, and the fact that some of his social acquaintances work as exotic dancers.

Michelle Bella told the jury she received a text message from Murray at 8:35 a.m. on June 25, 2009, the day Jackson died.

Another girlfriend, Bridgette Morgan, testified Monday that she called Murray at 11:26 a.m. on June 25. She said he did not answer his phone.

At the time Ms. Morgan called, Murray was participating in a 32-minute phone call with his Las Vegas office. That call had begun at 11:18 a.m.

At 11:49 a.m., Murray left a voicemail message for one of his patients in Las Vegas.

Two minutes later, Murray called Anding in Houston. It was during that call that Murray apparently discovered a problem with Jackson.

Murray’s call to the fourth girlfriend was made from the ambulance as a lifeless and unresponsive Jackson was being transported to UCLA Medical Center. Murray called Nicole Alvarez, with whom he shared a four-month-old son.

“I remember him telling me that he was on the way to the hospital in the ambulance with Mr. Jackson and not to be alarmed,” she testified. “He didn’t want me to be worried because he knew I would learn this through the news.”

Ms. Alvarez’s testimony was also important because she verified receiving seven separate shipments from April to June 2009 at her apartment from a Las Vegas pharmacy.

The owner of the pharmacy later told the jury that the shipments included 255 vials of propofol.

Prosecutors have said Murray purchased more than four gallons of the anesthetic for Jackson.